vc4: Move all of our fixed function fragment color handling to NIR.

vc4: Move all of our fixed function fragment color handling to NIR.

This massively reduces our dependency on VC4-specific optimization passes.

shader-db:
total uniforms in shared programs: 32077 -> 32067 (-0.03%)
uniforms in affected programs:     149 -> 139 (-6.71%)
total instructions in shared programs: 98208 -> 98182 (-0.03%)
instructions in affected programs:     2154 -> 2128 (-1.21%)